280 THE SIXTH DAY. [CHAP. 



embryo, and is thus completed (Fig. 9). The yolk-sac 

 now, like the allantois which closely wraps it all round, 

 lies loose in a space bounded outside the body by the 

 serous membrane, and continuous with the pleuro- 

 peritoneal cavity of the body of the embryo. Deposits 

 of urates now become abundant in the allantdic fluid. 



The loose and flaccid walls of the abdomen enclose 

 a space which the empty intestines are far from filling, 

 and on the nineteenth day the yolk-sac, diminished 

 greatly in bulk but still of some considerable size, is 

 withdrawn through the somatic stalk into the ab- 

 dominal cavity, which it largely distends. Outside the 

 embryo there remains nothing now but the highly 

 vascular allantois and the practically bloodless serous 

 membrane and amnion. The amnion, whose fluid during 

 the later days of incubation rapidly diminishes, is con- 

 tinuous at the umbilicus with the body-walls of the 

 embryo. The serous membrane (or outer primitive 

 amniotic fold) is by the completion of the cleavage of 

 the mesoblast and the invagination of the yolk-sac, 

 entirely separated from the embryo. The cavity of the 

 allantois by means of its stalk passing through the um- 

 bilicus is of course continuous with the cloaca. 



In the EMBRYO itself a few general points only de- 

 serve notice. 



By the sixth or seventh day the flexure of the 

 body has become less marked, so that the head does 

 not lie so near to the tail as on the previous days ; at 

 the same time a more distinct neck makes its ap- 

 pearance. 



Though the head is still disproportionately large, its 

 growth ceases to be greater than that of the body. 



